852 THE ORGANS OF SPECIAL SENSE. 



The skin lining the meatus is very thin, adheres closely to the cartilaginous 

 and osseous portion of the tube, and covers the surface of the membrana tympani, 

 forming its outer layer. After maceration the thin pouch of epidermis, when 

 withdrawn, preserves the form of the meatus. In the thick subcutaneous tissue 

 of the cartilaginous part of the meatus are numerous ceruminous glands, which 

 secrete the ear-wax. They resemble in structure sweat-glands, and their ducts 

 open on the surface of the skin. 



Relations of the Meatus. In front of the osseous part is the condyle of the 

 mandible, which, however, is separated from the cartilaginous part by the retro- 

 mandibular part of the parotid gland. The movements of the jaw influence to some 

 extent the lumen of this latter portion. Behind the osseous part are the mastoid 

 air-cells, separated from it by a thin layer of bone. 



The arteries supplying the meatus are branches from the posterior auricular, 

 internal maxillary, and temporal. 



The nerves are chiefly derived from the auriculo-temporal branch of the inferior 

 maxillary nerve and the auricular branch of the pneumogastric. 



Surface Form. At the point of junction of the osseous and cartilaginous portions the tube 

 forms an obtuse angle, which projects into the tube at its antero-inferior wall. This produces a 

 sort of constriction in this situation, and renders it the narrowest portion of the canal an im- 

 portant point to be borne in mind in connection with the presence of foreign bodies in the ears. 

 The cartilaginous is connected to the bony part by fibrous tissue, which renders the outer part 

 of the tube very movable, and therefore by drawing the pinna upward and backward the canal 

 is rendered almost straight. At the external orifice are a few short, crisp hairs which serye to 

 prevent the entrance of small particles of dust, or flies or other insects. In the external auditory 

 meatus the secretion of the ceruminous glands serves to catch any small particles which may 

 find their way into the canal, and prevent their reaching the membrana tympani, where their 

 presence might excite irritation. In young children the meatus is very short, the osseous part 

 being very deficient, and consisting merely of a bony ring (anmthis tympanicns), which supports 

 the membrana tympani. In the foetus the osseous part is entirely absent. The shortness of the 

 canal in children should be borne in mind in introducing the aural speculum, so" that it be not 

 pushed in too far, at the risk of injuring the membrana tympani ; indeed, even in the adult the 

 speculum should never be introduced beyond the constriction which marks the junction of the 

 osseous and cartilaginous portions. In using this instrument it is advisable that the pinna 

 should be drawn upward, backward, and a little outward, so as to render the canal as straight as 

 possible, and thus assist the operator in obtaining, by the aid of reflected light, a good view of 

 the membrana tympani. Just in front of the membrane is a well-marked depression, situated 

 on the floor of the canal and bounded by a somewhat prominent ridge ; in this foreign bodies 

 may become lodged. By aid of the speculum, combined with traction of the auricle upward and 

 backward, the whole of the membrana tympani is rendered visible. It is a pearly-gray mem- 

 brane, slightly glistening in the adult, placed obliquely, so as to form with the floor of the meatus 

 a very acute angle, (about 55) while with the roof it forms an obtuse angle. At birth it is more 

 horizontal, situated in almost the same plane as the base of the skull. About midway between 

 the anterior and posterior margins of the membrane, and extending from the centre obliquely 

 upward, is a reddish-yellow streak ; this is the handle of the malleus, which is inserted into the 

 membrane. At the upper part of this streak, close to the roof of the meatus, a little white 

 rounded prominence is plainly to be seen ; this is the processus brevis of the malleus, projecting 

 against the membrane. The membrana tympani does not present a plane surface ; on the con- 

 trary, its centre is drawn inward, on account of its connection with the handle of the malleus, and 

 thus the external surface is rendered concave. 



The Middle Ear, or Tympanum. 



The middle ear, or tympanum, is an irregular cavity, compressed from without 

 inward, and situated within the petrous bone. It is placed above the jugular fossa ; 

 the carotid canal lying in front, the mastoid cells behind, the meatus auditorius 

 externally, a.nd the labyrinth internally. It is filled with air, and communicates 

 with the naso-pharynx by the Eustachian tube. The tympanum is traversed by a 

 chain of movable bones, which connect the membrana tympani with the labyrinth, 

 and serve to convey the vibrations communicated to the membrana tympani across 

 the cavity of the tympanum to the internal ear. 



The tympanic cavity consists of two parts : the atrium or tympanic cavity proper, 

 opposite the tympanic membrane, and the attic or epitympanic recess, above the 

 level of the upper part of the membrane ; the latter contains the upper half of the 



